cli/request: support passing instance parameters from a file

Support a syntax like:

     slapos request --node=computer_guid=local --parameters=_=@~/request.json ERP5 https://lab.nexedi.com/nexedi/slapos/raw/1.0.145/software/erp5/software.cfg

to request an instance with a parameter named _ whose value is the content of
~/request.json file.

This also adds missing tests for slapos request command
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python2

01:25:58